In the physiology of visual receptor cells, the processes intervening between the absorption of photons by a visual pigment and the change in membrane voltage (the receptor potential) are unknown. In general, receptor potentials are thought to be generated by changes in the ionic conductance channels of the receptor cell membrane; light induces the release of some intracellular transmitter substance which modulates the ionic conductance channels. For invertebrate photoreceptors, a change of intracellular free calcium has been proposed to mediate light-adaptation. We propose to determine the relationship between intracellular free calcium and the sensitivity of the photoreceptor using the photoprotein aequorin.